"But don't forget he had a difficult pre-season - he was coming (back to Everton) late (after Euro 2016).

"I think now he is close to his best performance level and we will see that."

While managing to keep hold of Lukaku - who earlier this week was quoted by Belgian newspaper Het Nieuwsblad as claiming Juventus had been an option for him - during the summer transfer window, Everton also boosted their attack.

Winger Yannick Bolasie joined from Crystal Palace on August 15, and Everton also brought in forward Enner Valencia on loan from West Ham on deadline day.

Although the Valencia deal came very late in the window, Koeman says a potential move for the 26-year-old Ecuador international had been on the agenda for some time.

"Valencia was interested in coming to Everton, we were interested in him and already, earlier on, he was one of the options to get in a new front player," Koeman said.

"We know he can play like a nine, or behind a striker. We have different options with Enner, and that is good to have.

"He will bring that competition that all players need. Now, including Valencia, we have seven players for the three front positions in the team, and you need that - it is a long and difficult season.

"They need to give competition and the best ones in my opinion will start. They need to accept that and fight for the best chance to play."

Valencia could make his Everton debut in Monday's clash, while defender Seamus Coleman is ready for his Toffees comeback having recovered from an ankle problem and featured for the Republic of Ireland during the international break.

Darron Gibson and Tom Cleverley should also be available again after recent knocks, but fellow midfielder James McCarthy remains out after undergoing groin surgery.